Delayed sleep phase syndrome (DSPS) is a type of sleep disorder characterized by marked difficulty falling asleep at night and marked difficulty waking up in the morning. When sleep occurs, it is normal in content and duration. Thus DSPS is a disorder of the timing, not the quality, of sleep. It is hypothesized that DSPS arises from a disturbance in the biological clock that regulates the timing of sleep. The normal human sleep-wake cycle is controlled by a biological clock that in the absence of external influences generates a 25-hour rhythm. The timing of this circadian rhythm ordinarily is controlled by periodic stimuli in the environment that have the properties of time cues (zeitgebers), so that it maintains (1) a period of 24 hours and (2) a characteristic phase relationship to the day-night cycle (i.e., sleep occurs every 24 hours, and mainly at night). Throughout biology light (e.g., sunrise, sunset) is the most important zeitgeber. In theory DSPS could be caused by (1) an abnormally slow intrinsic rhythm of the sleep-wake cycle pacemaker (e.g., 25.5-26 hours) or (2) an abnormal response to zeitgebers, such as light. The purpose of this project is to investigate these two possible causes of DSPS: (1) the intrinsic rhythm of DSPS patients will be measured by asking them to live for at least two weeks in isolation from external time cues in special experimental rooms on the 4-West research unit. During the study motor activity and rectal temperature will be monitored continuously by computer. Circadian rhythm periods significantly longer than 25 hours would implicate a slow pacemaker as a cause of DSPS; (2) DSPS patients' hypothalamic sensitivity to light will be measured by investigating the degree to which nocturnal secretion of melatonin can be suppressed by varying intensities of light. Abnormal sensitivity to light would indirectly implicate altered sensitivity to zeitgebers as a cause of DSPS. Responses to light will also be evaluated using conventional optical and electrophysiological techniques.